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correspondingly long periods if the accumulation is not rapid. 

 Hence, although the accessions be few in any brief period the 

 final accumulation may be relatively great. When it is consid- 

 ered that the outer field within the earth's control which might 

 be traversed by such orbital molecules is 20,000,000 times as 

 great as that which is occupied by the collisional atmosphere, 

 it will be seen that the total mass of orbital molecules might be 

 considerable though the density remained very small. 



It will be observed that we have assigned to this large outer 

 field of the earth's control, two sets of moving molecules : the 

 fountain-like system of outshooting-returning molecules, and the 

 orbital system. The molecules of these two systems cut one 

 another roughly at right angles, and collision between them is 

 a necessary consequence. The logic of the case seems to drive 

 us to the conclusion that the collisions between the outshooting 

 and infalling molecules of the fountain-like system, among them- 

 selves, will divert into the orbital system molecules until such 

 numbers accumulate in orbital courses that as many of these lat- 

 ter are thrown out of their orbits by collisions with one another 

 or with the outshooting-infalling molecules as are thrown into 

 the system by the latter. In other words, we seem forced to rec- 

 ognize a self-imposed limit which marks a state of equilibrium 

 between the systems, after which as many orbital molecules, 

 roundly, will be thrown back into the collisional system, or out- 

 side the control of the earth, as are thrown into the orbital 

 system. Similar relations of equilibrium will be established mu- 

 tually between all of the three systems. 



We have thus far ignored the rotation of the earth. As the 

 atmosphere is carried with the earth the molecules in the outer 

 part of the equatorial atmosphere are moving from west to east 

 at a rate, roundly, of 1,000 miles per hour faster than the mole- 

 cules at the poles. It follows from this that, of the molecules 



